Relaxation of Photoexcited Electrons through Transition Dipole Coupling in the Hybrid Nanocomposite Porphyrin/Ag/TiO₂

For photosensitized semiconducting nanocomposites, multiple photosensitizers (e.g., metal plasmons and organic dyes) have been explored as a means of enhancing the light harvesting efficiency. In this work, we investigate the electron dynamics of a representative porphyrin─metal nanoparticle─semiconductor nanocomposite, in which only one of the sensitizers is able to interact with the semiconductor. Specifically, a nanocomposite composed of the J-aggregates of meso-Tetrakis (4-sulfonatophenyl)-porphyrins (TSPP), silver nanoplatelets (Ag NPs), and titanium dioxide (TiO2) nanorods was studied using ultrafast transient IR absorption spectroscopy. Both the Ag NPs and TSPP are capable of absorbing visible light near 400 nm and producing photoexcited electrons. However, while the Ag NPs can inject these hot electrons into the neighboring TiO2 semiconductors, the TSPP cannot because it does not physically interact with TiO2 nanorods. To describe the observed transient IR absorption traces, a complex physical model was developed, consisting of all likely dynamic processes between each component of the composite. It was deduced that in addition to electron injection from the Ag NPs to TiO2, a plasmon-induced resonance energy transfer (PIRET) mechanism resulting from transition dipole coupling was needed to account for the interaction between the two photosensitizers, Ag and the TSPP. The PIRET rate was deduced to be on the femtosecond time scale, which is faster than the hot electron injection rate from Ag to TiO2. Consequently, the TSPP is shown to act as an excitation sink in this nanocomposite, in which most photoexcited electrons are deactivated with their excitation energy funneled into the porphyrin. The observations and deductions made here provide lessons for the design of multiple light absorbers in photosensitized devices.